Optical materials have long been recognized as valued additions to secure articles (e.g., documents, labels, cards). These materials allow for a variety of self-authenticating optical effects thus rendering the secure article more resistant to counterfeiting.
By way of example, U.S. Pat. No. 7,333,268 to Steenblik et al. depicts a security device in the form of a micro-optic film material or structure that employs a regular two-dimensional array of lenses to enlarge micro-images. The film material or structure comprises: (a) one or more optical spacers; (b) an array of image icons positioned on one surface of the optical spacer; and (c) an array of microlenses positioned on an opposing surface of the optical spacer. The images projected by this film structure show a number of visual effects including orthoparallactic movement.
A common form of microlens enhanced surface that may be used with such film structures is a lenticular lens sheet. The lenticular lens sheet comprises a substrate with a top surface having a side-by-side array of substantially parallel refractive optical ridges (or substantially parallel reflective optical valleys) and with a bottom surface that is generally flat.
As is well known to those skilled in the art, a most important factor when designing or selecting a micro-optic security device for the security protection of an article is the security device's resistance to simulation by attempts at all levels of sophistication. Resistance to simulation is best done with methods that remain simple and obvious enough for the public to continue to be the main line of defense. The use of simple lens arrays such as packed hexagonal, square spherical and parallel cylindrical (lenticular) arrays may invite attempts to simulate the complex optical effects using commercially available lens sheets. Lenticular lens sheets are readily available up to about 200 lenses per inch (LPI) or 79 lenses per centimeter (LPCM), and are greater than or equal to approximately 125 microns in total thickness. While the thickness of these commercial lenticular lens sheets is greater than the thickness of lens layers used in a majority of micro-optic security devices for article security, the resolution of these 200 LPI lenticular lens sheets may be sufficient to satisfy the observer. It has therefore been difficult in practice to provide a highly counterfeit-resistance micro-optic security device that utilizes a lenticular lens array.